Aromatic Chemistry

Question 1

What is the molecular formula for benzene?

Answer 1

C_6H_6

Question 2

Describe the bonding in benzene

Answer 2

Each carbon atom uses 3 of its 4 electrons in the outer shell to form 3 single covalent bonds with 2 other carbon atoms and 1 hydrogen atom.
Each C atom has 1 unused electron in the p-orbital at a right angle to the plane of bonded C and H atoms.
The 6 spare electrons in the p orbitals are delocalised, so overlap to form a ring structure above and below the plane of carbon atoms

Question 3

What is the length of C-C bond and what does it tell us about the shape of benzene?

Answer 3

0.139nm - hence regular hexagon

Question 4

What is the evidence against the existence of the Kekule structure?

Answer 4

1. Did no undergo electrophilic addition when reacted with bromine, so no double bond in the structure.
   2.C-C bond length in benzene are different than in Kekule - C-C and C=C have different lengths in the Kekule structure, so forms irregular structure.
2. Enthalpy of hydrogenation

Question 5

What is the IUPAC name for the Kekule structure of benzene?

Answer 5

Cyclohexa-1,3,5-triene

Question 6

How is the enthalpy of hydrogenation evidence against the Kekule structure?

Answer 6

Enthalpy of hydrogenation of cyclohexa-1,3,5-ene is -360kJ/mol. The enthalpy for benzene is -208kJ/mol. Difference in enthalpies = -208–360 = 152kJ/mol. Hence benzene is 152kJ/mol less exothermic than cyclohexa-1,3,5-ene, meaning benzene is more stable than cycohexa-1,3,5-ene. The stability is due to the fact that the 6 spare electrons in the p orbital in benzene are delocalised to form a ring.

Question 7

Why are there no alternating single and double bonds in benzene?

Answer 7

The delocalisation of electrons means that there aren’t alternating double and single bonds

Question 8

Why is benzene more stable than cyclohexa-1,3,5-ene

Answer 8

Due to delocalisation of p electrons in benzene

Question 9

Describe nitration of benzene

Answer 9

Condition: 55 C + reflux
Type of reaction: Nitration
Mechanism: Electrophilic substitution
Reagents and conditions: HNO3 + conc. H2SO4

Question 10

How is the nitronium ion produced?

Answer 10

HNO3 + H2SO4 -> NO2+ + H2O + HSO4-

Question 11

Describe acylation of benzene

Answer 11

Type of reaction: Acylation
Mechanism: Electrophilic Substitution
Reagent: Acyl chloride
Condition: AlCl3 (catalyst)
Electrophile: CH3CO+

Question 12

What is the equation for the formation of the electrophile in the acylation of benzene?

Answer 12

CH3COCl + AlCl3 -> AlCl4- + CH3CO

Question 13

Describe the shape of benzene

Answer 13

Planar, 120 degree bonds around the carbon, C-C bonds are of equal length